Roberts and colleagues analyzed data from 672 participants in the Mayo Clinic Study of Aging. At the start, none of the participants had dementia, and they weren’t in hospice or terminally ill. Residents from Olmsted County, Minnesota, entered the study in 2004, at ages 70 to 89.

Participants described their diets in a survey and underwent tests for memory, executive function, language, visual-spatial skills and cognitive impairment. Researchers also used magnetic resonance imaging (MRI) to measure the cortical thickness of several regions of the brain.

Roberts and colleagues found that elderly patients with higher Mediterranean diet scores had higher cortical thickness in all lobes in the brain. Higher legume and fish intake, in particular, was associated with greater thickness, they reported July 25 in the journal Alzheimer’s and Dementia.

The study didn’t track patients long enough to see whether they actually developed any cognitive problems later on, however.

Yian Gu, an epidemiologist at Columbia University Medical Center in New York City, pointed out to Reuters Health that the study can’t show whether diet actually causes less brain atrophy.

Gu wasn’t involved with the new study, but she and her team have found ties between the Mediterranean diet, brain volume and total brain matter in their own research.

But “these are observational studies, not clinical trials, so a causal relationship can’t be established,” Gu said.

It’s possible, for example, that changes in brain structure result in poorer dietary habits, Gu said.

“As many people know, we don’t have a cure for Alzheimer’s disease, and there is a long period of time before onset. It’s important to find lifestyle factors that could prevent or delay the disease,” said Gu.

Although a doctor can’t prescribe a Mediterranean diet to elderly patients, it doesn’t hurt to follow it, Gu said.

“Specifically,” noted Roberts, “a high intake of fish, vegetables and legumes are beneficial, whereas a high intake of simple sugars and carbohydrates may have adverse effects on the brain.”